All right. You can do this with events fairly easily. The only problem is that it's kind of hidden. Go to page two of the event commands, and select "Change Map Settings...". From there, click the radio button next to "Fog Graphic", and click the arrow on the end of the selection box. That opens up a page where you can select a fog graphic (by choosing one from the menu to the right), set its opacity, blending (Add means that it adds to the colors below, making them brighter, and Sub does the opposite, making them dimmer, and looks rather like a negative). You can also set the zoom (100 is normal size) and the scroll direction. (By setting the number of pixels it scrolls along the x (SX) and y (SY) axes every frame)